1. Field of the Invention
The invention relates to an additive combination of (a) an ethylene backbone oil soluble polymer with (b) an oil-soluble polyester material, e.g., a homopolymer or copolymer of esters of acrylic or methacrylic acid, wherein at least 10 wt.% of said polymer is derived from an ester having substantially straight chain C.sub.4 to C.sub.16 alkyl groups extending from ester linkages. This combination is particularly useful in middle distillate fuel oils containing a fraction boiling above 370.degree. C., for controlling the size of wax crystals that form at low temperatures.
2. Description of the Prior Art
Various polymers, useful as middle distillate pour point depressants, prepared from ethylene have been described in the patent literature. These pour depressants include copolymers of ethylene and vinyl esters of lower fatty acids such as vinyl acetate (U.S. Pat. No. 3,048,479); copolymers of ethylene and alkyl acrylate (Canadian Pat. No. 676,875); terpolymers of ethylene with vinyl esters and alkyl fumarates (U.S. Pat. Nos. 3,304,261 and 3,341,309); polymers of ethylene (British Pat. Nos. 848,777 and 993,744); chlorinated polyethylene (Belgian Pat. No. 707,371 and U.S. Pat. No. 3,337,313); etc.
Polymers having alkyl groups in the range of C.sub.6 to C.sub.18, such as homopolymers and copolymers of olefins, alkyl esters of unsaturated dicarboxylic acids (e.g., copolymers of dialkyl fumarate with vinyl acetate), and copolymers of olefins and said esters, are known in the art, principally as lube oil pour depressants and/or V.I. improvers. For example, U.S. Pat. No. 2,379,728 teaches olefin polymers as lube pour depressants; U.S. Pat. No. 2,460,035 shows polyfumarates; U.S. Pat. No. 2,936,300 shows a copolymer of dialkyl fumarate and vinyl acetate; while U.S. Pat. No. 2,542,542 teaches copolymers of olefins, such as octadecene with maleic anhydride esterified with alcohol, e.g., lauryl alcohol, in lube and heating oils.
Synergistic pour point depressing combinations of various members of the above-noted two types of polymers in heavy fuels, e.g., residua and flash distillate fuels, which fuels contain relatively large amounts of waxes having 20 or more carbon atoms, is taught in U.S. Pat. No. 3,726,653. The cold flow of middle distillate fuels is improved by the addition combination of low M.sub.n ethylene copolymers such as ethylene-vinyl acetate and the polymer of a lauryl acrylic acid ester according to U.S. Pat. No. 3,275,427.